Artificial disc replacements (ADRs) are frequently made of hydrogels or metal and rubber. Hydrogel ADRs generally surround the hydrogel core with a flexible constraining jacket, as shown in PCT/USOO/80920, WO 00/59412.
Unfortunately, the flexibility of the hydrogel and the constraining jacket allow hydrogel ADRs to change shape and extrude through defects in the annulus through which the ADR was inserted, for example. Metal and rubber ADRs often fail at the metal-rubber interface. The rubber fails with the high shear stresses or the rubber separates from the metal with shear stress.
There does exist issued patents that relate to enclosing or sealing hydrogel materials. Of interest is U.S. Pat. No. 6,022,376, which teaches a hydrogel enclosed by a fluid permeable bag. However, the fluid bag does little to protect the hydrogel from shear stress, and the rough texture of the bag may cause hydrogel wear from friction.
U.S. Pat. No. 5,002,576 teaches an elastomer enclosed by rigid cover plates and a corrugated tube. The elastomer is sealed from fluids of the body. The corrugated tube allows movement of the cover plates. The corrugated tube may reduce shear forces on the elastomer. U.S. Pat. Nos. 5,865,846; 6,001,130; and 6,156,067 teach a spherical articulation between ADR EPs and an elastomer. The elastomer may be sealed within the ADR EPs. An annular gasket may reduce shear forces on the elastomer. U.S. Pat. No 5,893,889 teaches an elastomer that is sealed between ADR EPs. The device uses a ball and socket feature to reduce shear on the elastomer. U.S. Pat. No. 6,063,121 incorporates X-shaped wires into the '889 device to reduce rotation.